Clean up obsolete CalDAV environment variables

## Removed Obsolete Environment Variables:
- `CALDAV_SERVER_URL` - provided by user login
- `CALDAV_USERNAME` - provided by user login
- `CALDAV_PASSWORD` - provided by user login
- `CALDAV_TASKS_PATH` - not used in any features

## Kept with Intelligent Discovery:
- `CALDAV_CALENDAR_PATH` - optional override, defaults to smart discovery

## Changes:
### Backend
- Remove `CalDAVConfig::from_env()` method (not used in main app)
- Add `CalDAVConfig::new()` constructor with credentials
- Remove `tasks_path` field from CalDAVConfig
- Update auth service to use new constructor
- Update tests to use hardcoded test values instead of env vars
- Update debug tools to use test credentials

### Frontend
- Remove unused `config.rs` file entirely (frontend uses backend API)

## Current Authentication Flow:
1. User provides CalDAV credentials via login API
2. Backend creates CalDAVConfig dynamically from login request
3. Backend tests authentication via calendar discovery
4. Optional `CALDAV_CALENDAR_PATH` env var can override discovery
5. No environment variables required for normal operation

This simplifies deployment - users only need to provide CalDAV
credentials through the web interface, no server-side configuration required.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/handlers/auth.rs

@@ -52,13 +52,11 @@ pub async fn login(
     println!("📝 Created CalDAV config");
     
     // First verify the credentials are valid by attempting to discover calendars
-    let config = CalDAVConfig {
-        server_url: request.server_url.clone(),
-        username: request.username.clone(),
-        password: request.password.clone(),
-        calendar_path: None,
-        tasks_path: None,
-    };
+    let config = CalDAVConfig::new(
+        request.server_url.clone(),
+        request.username.clone(),
+        request.password.clone()
+    );
     let client = CalDAVClient::new(config);
     client.discover_calendars()
         .await